Fluxionality of [(Ph 3P) 3Rh(X)]: The extreme case of X = CF 3

Jenni Goodman, Vladimir V. Grushin, Roman B. Larichev, Stuart A. Macgregor, William J. Marshall, D. Christopher Roe

Research output: Contribution to journalArticlepeer-review


[(Ph 3P) 3Rh(F)] reacts with CF 3SiMe 3 to produce trans-[(Ph 3P) 2Rh(CF 2)(F)] (1; X-ray), which is equilibrated with a number of species in solution. Addition of excess Ph 3P shifts all of the equilibria to [(Ph 3P) 3Rh(CF 3)] (2; X-ray) as the only NMR-observable and isolable (84%) species. Complex 2 is uniquely highly fluxional in solution, maintaining ligand exchange even at -100°C (12.1 s -1). Activation parameters have been determined (variable- temperature 31P NMR) for the similar but slower exchange in the Me analogue of 2, [(Ph 3P) 3Rh(CH 3)]: E a = 16.5 ± 0.6 kcal mol -1, ΔG = 12.9 kcal mol -1 (calculated at -30°C), ΔH = 16.0 ± 0.6 kcal mol -1, and ΔS = 12.8 ± 2.3 e.u. Intramolecular exchange in [(R 3P) 3Rh(X)] occurs (DFT, MP2//BP86) via a distorted trigonal transition state (TS) with X in an axial position trans to a vacant site. The rearrangement is governed by a combination of steric and electronic factors and is facilitated by bulkier ligands on Rh as well as by strongly donating X that stabilize the TS. The Rh atom in [(H 3P) 3Rh(X)] has been shown to be more negatively charged (NPA) for X = CF 3 than for X = CH 3, despite the strongly oppositely charged carbon atoms of the CF 3 (+0.79e) and CH 3 (-0.96e) ligands. Clarification of stereochemical rigidity (X = halide, CN, OR, NR 2) versus fluxionality (X = H, Alk, Ar, CF 3) is provided, along with a resolution of the long-standing contradiction between the electronwithdrawing effect of CF 3 in organic compounds and its strong trans influence (electron donation) in metal complexes.

Original languageEnglish
Pages (from-to)4236-4238
Number of pages3
JournalJournal of the American Chemical Society
Issue number12
Publication statusPublished - 1 Apr 2009


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